docs: def set_scale(self):
          """generate the lookup which translates y-coordinate to fft-bin"""
  
          f_min = float(self.lower_freq)
          f_max = float(self.higher_freq)
          y_min = f_min
          y_max = f_max
          for y in range(self.image_height):
              freq = y_min + y / (self.image_height - 1.0) * (y_max - y_min)
              fft_bin = freq / f_max * (self.fft_size / 2 + 1)
              if fft_bin < self.fft_size / 2:
                  alpha = fft_bin - int(fft_bin)
                  self.y_to_bin.append((int(fft_bin), alpha * 255))